THE OPALIXID CILIATE INFUSOEIANS. 9 



contain what 1 have called endosarc spherules (black in figure 3), 

 generally a good deal smaller than the ectosarc spherules and of a 

 very different nature from the latter, as is indicated both by' their 

 structure and by their reaction to stains. In Protoopalina intestinals 

 they may be ellipsoidal, or dumbbell-shaped. Heavily stained with 

 iron-hematoxylin and strongly decolorized, the endosarc spherules 

 show a definite structure, appearing reticular, but probably indi- 

 cating that they are themselves alveolated. The nature and function 

 of the endosarc spherules is not known. It has been suggested that 

 they are nutritive plastids ^ similar to some of the paramylon bodies 

 in plants. Throughout the stroma of the endosarc there are numer- 

 ous granules, cytomicrosomes, apparently like those in the ectosarc. 



Along the axis of the body, in the endosarc, lies the more or less 

 well developed excretory organ in the form of an elongated vacuole, 

 or often an elongated group of contiguous vacuoles.^ The excretory 

 pore is posterior, lying just to one side of the posterior tip of the 

 body. The pore is not a permanent aperture, but opens only for the 

 occasional extrusion of excreta. Frequently, however, one finds a 

 slight depression in the surface of the body, indicating the position 

 of the excretory pore. Immediately anterior to the pore the excretory 

 canal usually widens to form a chamber (fig. 28, a and &, page 53). 

 In front of this chamber the canal may sometimes be branched, but 

 more generally it runs forward as a single irregular tube to the level 

 of the posterior of the two nuclei. The canal, when in its full devel- 

 opment, invariably is in close relation to the two nuclei, either being 

 coiled spirally around them, or being branched at this level and the 

 two branches being spirally bent around the nuclei. In some individ- 

 uals the excretory canal extends beyond the nuclei, well toward the 

 anterior end of the body; in others it extends but little, if any, in 

 front of the anterior nucleus. 



The excretory canal is not as definite a structure as this description 

 might seem to indicate. In most individuals it is present, but in some 

 it is not seen. When present, it may be developed only posteriorly. 

 In some cases it is an irregular canal posteriorly, while farther for- 

 ward it appears as an irregular line of vacuoles contiguous but seem- 

 ingly not fused. It seems that the excretory canals or excretory vacu- 

 oles are formed at times and disappear at other times. The structure 

 seems to be developed at different times to different degrees according 

 to the physiological state of the animal. Even when largest and most 

 fully developed it has no considerable delimiting membrane, such as 

 Schubotz (1908) found in Pycnothrix^ its wall seeming to be little, 

 if any, more emphasized than that of any vacuole or than an ordi- 

 nary alveolar film. The excretory canal seems to be little more than 



8 See Metcalf (1909), p. 22, also p. 89. » See Metcalf (1907 h and c). 



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